effects in the shallow groundwater zone in weathered Sarnia-area clay. Can. Geotech. J. 26,. 43-56. Hvorslev, M. J. (1951) Time lag and soil permeability in ...
HydrologicalSciences -Journal- des Sciences HydrologUpies,3S,2,4/1993
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Effect of purging on hydraulic conductivity measured in piezometers installed in an aquitard CHRISTOPHER J. EVERTS Environmental Science and Engineering Inc., 8901 N. Industrial Road, Peoria, Illinois 61615, USA
RAMESHWAR S. KANWAR Department of Agricultural and Biosystems Systems Engineering, Iowa State University, Ames, Iowa 50011, USA
Abstract Piezometers and wells installed for water quality monitoring are frequently used to assess the saturated hydraulic conductivity QK) in the surrounding formation. A series of recovery tests was conducted to evaluate how purging, required to obtain representative water quality samples, affected measured values of hydraulic conductivity in 15 newly installed and undeveloped piezometers placed to between 2 and 15 m depth (in oxidized and unoxidized material) in a loamy glacial till (K range from 10"6 to 10"9 m s"1). Piezometers were purged between 9 and 11 times for sampling over a period of five months. The effect of the purgings on piezometer development was evaluated by changes in slope of the water level recovery curves which were used to calculate hydraulic conductivity. The first five purgings following piezometer installation increased K in the 15 piezometers by an average of 34%. The average increase in a value of K after 10 purgings was 44 %. Values measured for hydraulic conductivity in a 75 mm diameter auger hole appeared stable after four purgings but piezometers installed in larger diameter boreholes (100 mm to 280 mm) snowed increases in K with up to 10 purgings. The hydraulic conductivity determined for piezometers installed at a 30° angle to the vertical showed greater variability than was observed in the adjacent vertically installed piezometers at the same depth.
Les conséquences des opérations de purge sur la conductivité hydraulique mesurée dans des piézomètres installés dans la nappe aquifère Résumé Des piézomètres et des puits mis en place afin de contrôler la qualité de l'eau sont fréquemment utilisés pour estimer la conductivité hydraulique de saturation (K) dans les ensembles environnants. Une série de prélèvements a été effectuée afin d'évaluer dans quelle mesure les purges, nécessaires à l'obtention d'échantillons représentatifs de la qualité de l'eau, affectaient les valeurs mesurées de la conductivité hydraulique. Ce test a été réalisé dans 15 piézomètres récemment installés et jusqu'alors inexploités, placés à une profondeur variant entre 2 et 15 m (matériaux oxydés et non oxydés) dans un sol d'origine glaciaire (K étant compris entre 10"6 et 10*9 m s"1). Les piézomètres ont été purgés entre 9 et 11 fois sur une durée de 5 mois lors de la collecte d'échantillons. L'effet de ces purges sur l'exploitation des piézomètres a été évalué par les variations de pente des courbes estimant le niveau de l'eau après la purge, ces courbes ayant été utilisées pour calculer la conductivité Open for discussion until 1 October 1993
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Christopher J. Everts & Rameshwar S. Kanwar hydraulique. Les cinq premières purges suivant l'installation des piézomètres ont augmenté la valeur de K d'environ 34% dans les 15 piézomètres considérés. Après 10 purges, la valeur de K s'est en moyenne élevée de 44%. Les valeurs mesurées de la conductivité hydraulique, dans les trous de diamètre 75 mm, sont apparues stables après 4 purges mais des piézomètres mis en place dans des trous de diamètre plus large (de 100 mm à 280 mm) ont fait apparaître des augmentations de K jusqu'à la dixième purge. La conductivité hydraulique déterminée pour des piézomètres placés à un angle de 30° par rapport à la verticale s'est avérée d'une variabilité plus importante que celle observée dans le cas de piézomètres sensiblement verticaux à la même profondeur.
INTRODUCTION Public concern over existing and potential groundwater contamination has prompted an increasing number of studies to document the processes of groundwater flow and chemical transport through fine-grained till materials and to quantify recharge to water table and bedrock aquifer systems. Hydraulic conductivity is an important parameter in understanding those processes. It is frequently necessary and economical to use piezometers or wells installed for water quality monitoring to obtain information on hydraulic conductivity, either as an observation well for a pumping test, or more often as a location for a rising or falling head test. Hydraulic conductivity (K) values thus obtained, particularly from rising head tests, may be as much a function of installation procedures and conditions at the borehole wall or in the sand or gravel pack as representative of the actual hydraulic conductivity in the adjacent formation. Installation of monitoring piezometers in low conductivity materials can reduce the effects of secondary porosity by smearing the walls of the bore hole (D'Astous et al., 1989; Keller et al., 1986). Vonhof (1975) used slug tests to evaluate the extent of well development from air pumping, jetting and surging, and found that such procedures could increase transmissivity for a well in a confined aquifer from 50 to 90%. Full development of piezometers or wells installed in aquitard materials for monitoring by surging, over-pumping or other well development methods may not be feasible due to low conductivity, or may be undesirable because of concern for contamination, unrepresentative water quality samples, or excessive time or equipment requirements. The objective of this study was to evaluate how repeated piezometer purgings, necessary for water quality sampling, influence the measured saturated hydraulic conductivity in the region surrounding a piezometer installed in a till material. The effects of purging on changes in K were compared for piezometers installed with different borehole diameters, angled and vertical installation, installation depth and length of openings. MATERIALS AND METHODS Wisconsinan aged till deposited in the Des Moines lobe covers roughly one
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fifth of Iowa. A detailed description of glacial till properties found in the region is given by Lutenegger et al. (1983). Because of the importance of this till formation to the hydrogeology of the region, a 13 ha site was chosen for intensive monitoring to investigate in detail the hydraulic properties and potential for groundwater contamination of aquifers below the formation. The study field was located at the Iowa State University Agricultural Engineering Research Center, 13 km west of Ames, Iowa. Soil at the site is comprised of loess and oxidized till from the surface to a depth of 3 m. Underlying the surface oxidized layer is a layer of unoxidized till extending to 24 m. Particle size data determined from cores collected at Sites 1 and 7 and representative of the whole site are presented in Table 1 (Kanwar et al., 1989). Table 1 Particle size distribution from recovered soil cores Depth (m)
% Sand >0.05 mm
% Silt
% Clay — 1] so as to minimize any effect of groundwater drawdown in the vicinity of the piezometer. At Site 7, the five piezometers included in this investigation were each installed to a depth of 6.2 m with a distance of 2.3 m between piezometers. Piezometers, 7D, 7E, 7F, 7G and 7J were installed in boreholes made by 75, 100, 180, 280 and 180 mm diameter hollow stem augers, respectively (Table 2). As a means of reducing the migration of fines, a covering of nonwoven polypropylene geotextile was secured over the slotted openings of piezometer 7J prior to placement of the sand pack. The rate of water level recovery was recorded following the first, fourth, and ninth to eleventh time each piezometer at Site 7 was purged following installation.
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A single piezometer was installed at an angle 30° from the vertical at both Sites 1 and 3. The vertical depthsfromthe surface to the bottom of angled piezometers ID and 3H were 8.2 and 8.0 m respectively. Angled piezometers, ID and 3H, were paired with similarly installed vertical piezometers (1C and 3G) installed 4 m away. Both angled and vertical piezometers were each installed with a 180 mm diameter auger and a slotted screen length of 1.2 m. The purpose of installation of the two angled piezometers was to increase the probability of intercepting any vertical fractures present in the unoxidized till (D'Astousef a/., 1989). RESULTS AND DISCUSSION Effect of piezometer purgings on K values Hydraulic conductivity values calculated from all recovery tests performed on all 15 piezometers are given in Table 3. Figure 1 illustrates die variability observed in K resulting from repeated purging from the 15 piezometers. Variability is expressed as the ratio of K determined after a purging to the value for K obtained from the first purging (K/Klst). Piezometer development from the first five purgings after piezometer installation increased the measured value of K by an average of 34%. Five additional purgings to give a total of 10 added an additional 10% to die value of K. After an average of 10.7 purgings the values for K obtained from recovery tests of the 15 piezometers averaged 44% higher than the value for K determined following the first purging after installation. Interaction between purging effect, K, and piezometer depth Figure 2 shows graphically the relationship between the depth of piezometer placement and K observed from the 41 piezometers installed at the site. The R2 for the regression of depth versus log K down to a depdi of 10 m was 0.48, statistically significant at less than the 1 % level (32 degrees of freedom). The trend of decreasing conductivity with depth was also reported by Lutenegger (in Kanwar et al., 1989) from dilatometer measurements made near Site 1. The decrease in K with depth is most probably die result of fracture closure due to increasing overburden pressure (Prudic, 1982). Twelve of the 15 piezometers showed an increase in K with repeated purgings. The variability in K observed from repeated purgings (Fig. 1) prevented determinations of any statistically significant correlations. No correlation was found between purging response and K (Fig. 3) or between purging effect and piezometer deptii (Fig. 4). Piezometers at 5.9 m (3C) and 14.6 m (3F) showed a decrease in K with continued purging aldiough the data
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Fig. 3 Variation in the ratio of final to initial hydraulic conductivity resultingfromrepeated purgings of 15 piezometers. statistically significant. The trend would be consistent with decreasing velocities and lower turbulence at the sand pack/formation contact as the diameter of the auger hole increases. The first recovery test in piezometer 7D with a borehole diameter of 75 mm showed an unusually rapid increase in water level between 2 and 18.5 min (Fig. 6). This initially steep response (corresponding to a K 2.6 -X--
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Christopher J. Everts & Rameshwar S. Kanwar •
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Fig. 5 Influence of auger size on changes in K resulting from purging. value of 7.1 x 10~7 m s"1) was attributed to settlement of the sand pack or grouting adjacent to the screened area, a consequence of the difficulty in placing a uniform sand pack around a 50 mm PVC screen in a 75 mm borehole. Following the rapid initial recovery, water level recovery corresponded to a K value of 7.2 x 10"9 m s"1. Recovery tests made following the 4th and 9th purgings each resulted in recovery slopes corresponding to 1.7 x 10"8 m s"1 suggesting full development for the piezometer installed in the 75 mm diameter borehole was reached prior to the 4th purging. Piezometers installed with auger diameters greater than 75 mm showed that recovery slope and thus K increased with successive purgings. The average increase in K between the 4th and 10th to 11th purging for piezometers 7E, F and G (100, 180 and 280 mm borehole respectively) was 14%. The 180 mm borehole with geotextile covering the slotted opening (7J) was least influenced by repeated purging, showing a 1% increase in the observed K between the 1st and 9th purging. Piezometer 7F by comparison, installed with a 180 mm auger diameter but without a geotextile fabric showed an increase in K of 87% between the first and 10th purging. The value for K observed after ten purgings for piezometer 7F was 2.5 times higher than that observed in piezometer 7J installed with a geotextile fabric. The difference was probably the result of reduced migration and removal of fines during purging. Purging effect on angled piezometers and screen length Figure 7 compares changes in K for paired vertical and angled piezometers.
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Fig. 6 Semi-log recovery plots for piezometer 7D (75 mm diameter borehole) for the 1st, 4th and 9th purging. Each piezometer pair shows similarly shaped responses. However, a comparison of the response of the two angled piezometers shows a widely different response. The two vertical piezometers (1C and 3G) displayed a similar purging response after 6 purgings. The large increase in K observed between the 1st and 5th purging (153%) for angled piezometer 3H suggests fracture interception. Piezometers 3D and 3G located 7 m apart were both installed to a depth
5 6 7 Number of Purgings
- *- - 3H angled - H - 3G
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Fig. 7 Comparison of purging effect for vertical (1C and 3G) and angled (ID and 3H) piezometer installation.
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Christopher J. Everts & Rameshwar S. Kanwar
of 6.9 m depth. Piezometer 3D had a screen slotted length of 0.6 m compared to the 1.2 m screen length for piezometer 3G. Figure 8 shows that both screen lengths followed a similar trend with repeated purgings, with the 1.2 m screen length showing the greater variability.
3
4
5
6
7
8
Number of Purgings 3D 0.6 m screen - * - - 3G 1.2 m screen
Fig. 8 Comparison of purging effect by screen length for piezometers 3D (0.6 m openings) and 3G (1.2 m openings). CONCLUSIONS The sampling history of wells and piezometers located in till materials should be a consideration when interpreting hydraulic conductivity (K) measurements by rising or falling head tests. The effect of piezometer purging was primarily to increase values for K although the magnitude was not predictable and showed considerable variability. The effect of purging on the variability in K observed in the piezometers was attributed to three factors. Initial consolidation of the sand pack caused a reduction in the porosity. A second factor contributing to the increase in K observed from purgings was the continuous migration of fines into the borehole as a result of purging. This was evidenced by the milky appearance of water purged from many of the boreholes. A third factor which was believed to have contributed to the increases in K observed is that the secondary porosity in the form of fractures known to exist in the till became more significant as purgings continued. The extent of the contribution of secondary porosity to the initial water level recovery of a piezometer varied depending on damage and extent of smearing on the borehole walls during the initial piezometer installation as well as the variation in the number and size of fractures intercepted by each piezometer.
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This study yielded the following conclusions on piezometers installed in a loamy glacial till: (a) hydraulic conductivity increased with continued purgings in 27 out of 35 recovery tests conducted in 15 piezometers (Table 3). The average increase in K was observed in 15 piezometers, installed between 2.2 m and 14.6 m. The first five purgings after installation saw an average increase in K values of 34%. Average K increased by 44% between the first and tenth purgings. The effect of purging on K for individual wells ranged from a decrease of 26% between the first and eleventh purging (piezometer 3C) to a maximum increase of 240% between the first and ninth purging (piezometer 7D); (b) based on results of the 15 piezometers it is recommended that rising head tests should be conducted on wells or piezometers installed in till materials that have been purged a minimum of 10 times; (c) the increase in K resulting from purging (with the exception of piezometer 7E) decreased as borehole size increased. Geotextile fabric wrapped around the slotted openings of the piezometer also appeared to decrease the development effect of purging. Acknowledgements This project was supported in part by the Iowa Department of Natural Resources with funds provided from the Iowa Groundwater Protection Act. Journal Paper no. J-13936 of Iowa Agricultural and Home Economics Experiment Station, Ames, Iowa 50011, USA. Project no. 2898. REFERENCES Bouwer, H. (1989) The Bouwer and Rice slug test - an update. Ground Wat. 27(3), 304-309. Bouwer, H. & Rice, R. C. (1976) A slug test for determining hydraulic conductivity of unconfined aquifers with completely or partially penetrating wells. Wat. Resour. Res. 12(3) 423-428. Chapuis, R. P. (1989) Shape factors for permeability tests in boreholes and piezometers. Ground Wat. 27(5)647-653. D'Astous, A. Y., Ruland, W. W., Bruce, J. R. G., Cherry, J. A. & Gillham, R. W. (1989) Fracture effects in the shallow groundwater zone in weathered Sarnia-area clay. Can. Geotech. J. 26, 43-56. Hvorslev, M. J. (1951) Time lag and soil permeability in ground-water observations. Bull. no. 36, Waterways Experiment Station, US Army Corps of Engineers, Vicksburg, Mississippi, USA. Johnson, H. P., Frevert, R. K. & Evans, D. D. (1952) Simplified procedure for the measurement and computation of soil permeability below the water table. Agric. Engng. May, 283-286. Kanwar, R. S., Baker, J. L., Horton, R., Handy, R. L., Jones, L. C , Jones, & Lutenegger, A. J. (1989) Aquitard hydrology project Ames Research Site annual progress report 1988-1989. Contracted by Iowa Department of Natural Resources Geologic Survey Bureau, Iowa City, Iowa, USA. Keller, C. K., Van Der Kamp, G. & Cherry, J. A. (1989) A multiscale study of the permeability of a thick clayey till. Wat. Resour. Res. 25(11), 2299-2317. Lutenegger, A. 1 , Commas, T. J. & Hallberg, G. R. (1983) Origin and properties of glacial till and diamictons. ASCE Geotechnical Engineering Division Special Publications on Geological Environment and Soil Properties. ASCE Convention, Houston, Texas, USA, 17-21 October 1983,310-331. Prudic, D. E. (1982) Hydraulic conductivity of a fine-grained till, Cattaraugus County, New York. Ground Wat. 20(2), 194-204. Vonhof, J. A. (1975) Hydrodynamic response- or slug tests as a means to monitor the progress of well development. Can. Geotech. J. 12(1), 1-12. Received 22 April 1992; accepted 12 October 1992
